In a third-generation computerized tomography (CT) scanner, an X-ray source and a detector array are mounted to a disk assembly, which is rotatable about an object to be scanned. The rotating disk assembly is supported by a rigid, stationary gantry. The detector array comprises a plurality of channels extending laterally from a central channel. During a scan, the source and detectors scan the object at incremental rotation angles. A process referred to as reconstruction generates a series of two-dimensional images, or slices, of the object from the captured data.
FIG. 1 is an axial or front view of a third-generation X-ray CT scanner, including an X-ray source 20 and a detector array 22 mounted on a disk assembly rotatable about an object at an angular velocity of .omega.. The detector array 22 is typically disposed as a row in the shape of a circular arc, and centered about a point 26, referred to as the "focal spot", where X-ray radiation emanates from the X-ray source 20. The X-ray beam emanating from the focal spot and incident on the detector array therefore define a fan beam. A rotating coordinate system x'y'z', fixed relative to the disk assembly, is used in FIG. 1 and subsequent figures to illustrate the position of the focal spot of the X-ray source 20 with respect to the detector array 22. The z' axis coincides with the z axis of a stationary coordinate system (x,y,z). The x'y' plane is coplanar with the xy plane, and rotates about the z' axis at angular velocity .omega.. The source 20 and the detector array 22 lie on the x'y' plane, and the y' axis intersects the rotation center or isocenter 24 and the center channel 22A of the detector array. In a conventional fixed focal spot system, the focal spot position remains fixed on the y' axis throughout a scan.
For the purpose of discussion, the direction on the x'y' plane, i.e. within the rotation plane of the fan beam, parallel to, or substantially parallel to, the x' axis, is referred to herein as the "lateral" direction, and the direction parallel to, or substantially parallel to, the z' axis is referred to as the "longitudinal" direction.
It is well understood that oscillatory movement of the focal spot in the x'y' plane during a scan can improve the image-quality of a CT scanner, as described, for example, in U.S. Pat. No. 5,841,829, incorporated herein by reference. This is referred to as a "flying focal spot". In this configuration, the focal spot 26 is made to oscillate in a lateral direction, between positions a and b, as shown in FIG. 1. The displacement of the focal spot in the lateral direction, parallel to the x' axis is plotted with respect to time in FIG. 2 for a standard case of simple harmonic oscillation of the lateral movement.
During a scan, X-ray intensity is sensed at each detector channel over a range of rotation or view angles, as the disk assembly rotates at an angular velocity .omega., and while the X-ray source continuously irradiates the object being scanned. All detector signals are typically sampled at the same time at each rotation angle. The signal of the X-ray intensity incident on a detector is integrated or filtered over a short time duration. During this time duration, focal spot displacement varies slightly near the peak of the sinusoidal oscillation. The lateral positions a and b indicated in FIGS. 1 and 2 represent the mean displacements following the signal integration or filtration.
Oscillating focal spot displacement in the lateral direction, parallel to the x' axis, is equivalent to a tangential offset of the detector array. Usually, the flying focal spot is operated to have displacements a and b located at the equivalent of positive and negative quarter-detector offsets. Under these conditions, the data collected from the focal spot at position a are shifted by a half detector width with respect to the data collected from the focal spot at position b.
By alternating the focal spot between displacement positions a and b at successive rotation angles, the effective number of detectors is doubled. At alternate even and odd rotating angle increments, the data are sampled at the even and odd detection positions respectively, resulting in data that are interleaved in the sampling space. As a result, the spatial resolution of the reconstructed image is doubled. The flying focal spot is applicable to both step-and-shoot scanning and helical scanning.